Arrangement For Controlling Exhaust Pressure Pulses At An Internal Combustion Engine

ABSTRACT

An arrangement for controlling exhaust pressure pulsation in a combustion engine including six cylinders. The engine also includes an inlet for supply of air to the cylinders, and an exhaust manifold for delivery of exhaust gases from the cylinders. The manifold is provided with a first outlet to an exhaust system and with a second outlet to a conduit for feeding exhaust gases back, via an EGR circuit, from at least one of the engine&#39;s cylinders to the inlet. At least one of the cylinders is adapted to enrich the exhaust gases with unburnt hydrocarbon with a view to regeneration of an exhaust gas post-treatment unit situated in the exhaust system. A special configuration of the exhaust manifold results in separation of regeneration gas from EGR gas in the exhaust manifold.

BACKGROUND AND SUMMARY

The present invention relates to an arrangement for control of exhaustpressure pulsation of a combustion engine.

The diesel engine is known for operational reliability and low fuelconsumption but does not produce as low emissions as, for example, apetrol engine provided with a three-way catalyst. One way of improvingthe emissions from a diesel engine is to fit a particle filter whichfilters soot and particles from the exhaust gases and/or the NOxpost-treatment system. These filters are usually very effective andgather both large and small particles. To prevent the filter frombecoming full of soot and causing a major pressure drop for the exhaustgases leaving the engine, the soot has to be burnt. One method is forthis soot to be burnt by the nitrogen oxides contained in diesel exhaustgases. In that case, the portion of the nitrogen oxides that takes theform of NO₂ can oxidise the soot within the temperature interval ofabout 250 to 400° C., but this process takes a relatively long time andneeds to be more or less constantly active even if there is an oxidationcatalyst before the filter or the filter itself is covered with acatalytic layer. Another method for oxidising soot accumulated in theparticle filter is to heat the filter to about 600-650° C. so that thesurplus oxygen O₂ from diesel combustion can oxidise the soot directly,which is a rapid process. Soot accumulated over many hours of operatingtime can be oxidised away in a time of the order of 5 to 10 minutes. Theexhaust temperature of a diesel engine normally never reaches 600-650°C., particularly after a turbo unit whereby the turbine extracts powerfrom the exhaust flow and causes a temperature drop. It is not unusualthat the exhaust temperature after the turbine of the turbo unit isoften lower than 250° C., a temperature (tit which an oxidation catalystdoes not function. There are various special measures for temporarilyincreasing the exhaust temperature of a diesel engine.

Regenerating a NOx trap or a NOx catalyst requires hydrocarbon, e.g. inthe form of fuel which can for example be supplied by post-injectioninto one or more cylinders.

Some of the most modern diesel engines are often equipped with exhaustgas recirculation (EGR) to reduce emissions of nitrogen oxides.Combining this system with particle filters and/or NOx post-treatment byso-called NOx trap or NOx catalyst entails complications. When it isdesired to adopt measures intended, for example, to increase thetemperature of the exhaust flow, there is no point in increasing theexhaust temperature in the EGR flow, since it has to pass through theEGR cooler, whereupon the increased exhaust heat increases the load onthe EGR cooler without being used in any positive way but resultingrather in the fuel consumption being somewhat increased due to theincrease in the exhaust temperature. There are therefore gains to bemade with regard to fuel consumption by channelling increased exhausttemperatures to the exhaust system but preventing this temperatureincrease in the exhaust gases being recirculated in the form of cooledEGR. A deliberate increase in the exhaust temperatures of the variouscylinders can be achieved, for example, by ordinary combustion beingdelayed and/or post-injection being effected in such a way that theextra fuel added is burnt but only contributes to a small extent to theexpansion work in the cylinders.

The temperature of the exhaust flow before the particle filter can beincreased to 600-650° C. by oxidising (burning) fuel in the oxidationcatalyst fitted before the filter. This fuel can be injected in theexhaust line just before the catalyst, or extra injection can beeffected in the cylinder, but within a crankshaft angle interval inwhich the conditions for the fuel to ignite in the cylinder are notfulfilled. This is for example the case when the fuel is injected intothe cylinder during the latter part of the expansion stroke or duringthe exhaust stroke, so-called late post-injection. If the fuel isinjected into the cylinder, the same injection equipment can preferablybe used as for the ordinary fuel injection. This avoids the cost andcomplications of a further extra injector for the fuel which is to beoxidised in the oxidation catalyst. If the extra fuel is supplied bylate post-injection in all the cylinders and the engine at the same timeuses feedback of exhaust gases to the inlet (EGR), this means that partof the unburnt fuel intended for the catalyst reaches the EGR circuitand the inlet ducts. Any fuel vaporised when leaving the cylindersduring the exhaust stroke but later cooled in an EGR cooler can becondensed and part of the resulting liquid may possibly accumulate inrecesses/pockets during certain modes of operation. When the engine'soperating mode subsequently changes and the gas flow changes, the liquidaccumulated may possibly accompany the gas flow momentarily andimmediately enter one or more cylinders in an uncontrolled manner. Thisgives rise to uncontrolled behaviour of the engine and may cause seriousaccidents and/or damage. Fuel which does not condense but passes throughthe EGR cooler and the inlet ducts in a vaporised form also gives riseto altered combustion conditions. In a diesel engine, the fuel should infact not arrive at the same time as the inlet air, as the fuel supplytiming should be determined by the injection system. Even a small leakin the EGR circuit may become very obvious if condensed fuel begins totrickle out through the leak.

These adverse effects due to fuel in the EGR circuit can be preventedeither by closing the EGR circuit when late post-injection takes place,or alternatively, as a consequence of the NOx level increasing from thecombustion, applying late post- combustion only in carefully selectedcylinders in combination with designing the exhaust manifold in such away that EGR gases are only taken from cylinders which have no latepost- injection. U.S. Pat. No. 5,987,884 and U.S. Pat. No. 6,141,959describe how an exhaust system divided into two portions, combined withlate post-injection in certain designated cylinders, to avoid the aboveproblems caused by fuel in the EGR circuit. The method of dividing anexhaust system into two portions usually means that the exhaust systemis more expensive to manufacture and requires more space for installingit.

It is desirable to provide an arrangement in a combustion engine whichavoids the effects on the flow in the EGR circuit which are caused bymeans for regeneration of an exhaust post-treatment unit situated in theexhaust system even in the case of exhaust systems in which all theexhaust gases are collected in one and the same duct entirely outsidethe exhaust ports of each cylinder. It is also desirable to provideuniform pressure pulses at both the exhaust outlet of the exhaustmanifold and its EGR outlet.

BRIEF DESCRIPTION OF THE DRAWINGS

The arrangement according to the invention is described below in moredetail with reference to examples of embodiments depicted in theattached drawings, in which

FIG. 1 schematically depicts a combustion engine with an EGR circuit anda regenerable exhaust post-treatment unit according to the invention,

FIG. 2 schematically depicts an exhaust manifold according to a firstembodiment of the invention,

FIG. 3 in a corresponding manner depicts an exhaust manifold accordingto a second embodiment of the exhaust manifold,

FIG. 4 is a diagram representing the pressure at the turbine outlet ofthe exhaust manifold, and

FIG. 5 is a diagram representing the pressure at the outlet of theexhaust manifold to the EGR circuit.

DETAILED DESCRIPTION

The combustion engine 10 schematically depicted in FIG. 1 comprises anengine block 11 with six piston-cylinders 12 with an inlet manifold 13and an exhaust manifold 14. Exhaust gases are led via an exhaust passage15 to the turbine wheel 17 of a turbo unit 16. The turbine shaft 18drives the compressor wheel 19 of the turbo unit that compresses aircoming in via an inlet passage 20 and passes it on via a charge aircooler 21 to the inlet manifold 13. Fuel is supplied to the respectivecylinders 12 via (undepicted) injection devices.

Exhaust gases which have passed through the turbine unit 16 are led tothe atmosphere via the exhaust line 22, which leads the exhaust gasesthrough a regenerable device for exhaust post-treatment, e.g. a particletrap or NOx trap 23. Regeneration of the particle trap is effected bysupply of unburnt fuel from any of the cylinders 12, e.g. by so-called“post-injection”, which by oxidation of the fuel in the catalystupstream from the particle filter raises the temperature in the particletrap sufficiently for soot in the latter to ignite and burn away.

Exhaust gases are also led back to the inlet side of the engine via aconduit 24 as so-called EGR gas, in order to reduce the discharge ofnitrogen oxides from the engine by known technology. This line comprisesa valve 25 serving both as a one-way valve and as a control valve forregulating the EGR flow. There is also a cooler 26 for cooling the EGRgases.

The valve 25 is connected to an engine control unit 27 comprisingcontrol programmes and control data for controlling the engine on thebasis of input data. The engine control unit 27 is for example connectedto a sensor 28 that detects engine speed.

Here below follows a description of how admission of unburnt fuel to theEGR circuit is prevented despite the exhaust system not being dividedinto two separate portions and despite all the exhaust gases beingcollected in one and the same duct entirely outside the exhaust ports ofeach cylinder. The presupposition is a six-cylinder engine withconventional ignition sequence of 1-5-3-6-2-4. This also applies to aso-called V6 engine comprising a cylinder bank of three cylinders on oneside of the middle of the Engine and a cylinder bank of three cylinderson the other side of the middle of the engine, the two cylinder banksbeing connected by exhaust manifolds.

The ignition sequence which applies for a usual straight six-cylinderengine results in the exhaust pulses of the various cylinders comingalternately from the forward engine half (cylinders 1, 2, 3) and therear engine half (cylinders 4, 5, 6). With a V6 configuration, theexhaust pulses come alternately from the left bank of cylinders and theright bank of cylinders.

In the exhaust manifold of a six-cylinder engine provided with a turbounit with variable turbine geometry (VTG), the exhaust gases are mostcommonly led together from all six cylinders, since a VTG turbineusually has no dividing wall. A turbo unit for a six-cylinder enginewith fixed turbine often has a dividing wall, in which case it iscombined with an exhaust manifold which separates the gases in such away that alternate exhaust pulses take place in one half and the otherexhaust pulses in the other half. However, this type of separatedexhaust pulse arrangement is not employed in combination with a turbinethat has no dividing wall, e.g. a VTG turbine.

An exhaust manifold which collects the exhaust gases from all sixcylinders without any dividing wall to separate alternate exhaust pulsesmay be provided with EGR outlet at an outer end of the extent of theengine, and the turbine extraction may be situated preferably at theother end of the extent of the exhaust manifold. For such an exhaustmanifold, the pressure pulses may remain six approximately equalpressure pulses irrespective of whether one end is tapped by an outletfor EGR, even for EGR flows of the order of 10% to 50% (massflow_(EGR)/[mass flow_(EGR)+mass flow_(exhaust)])- If such an exhaustmanifold separates the exhaust flows from the two halves (forward/rear)of the engine while EGR is tapped from only one half, the result islimping pressure pulsation, i.e. pressure pulses significantly strongerin the half which supplies only the turbine with exhaust gases than thepressure pulses prevailing in the half which supplies gas flow to boththe EGR circuit and the turbine.

Limping pressure pulsation is less advantageous for the turbine, withthe result that the turbine operates in a working range of lowerefficiency. Such separation causes the EGR circuit to receive only threeexhaust pulses in two complete engine revolutions (a Whole four-strokeworking cycle), which is disadvantageous as compared with six smallerpulses, since, when the exhaust gases require mixing with fresh airbefore the gases fed back are led into the engine's inlet pipe, the gasmixture needs to be as homogeneous as possible both spatially andchronologically (a sparse pulsated flow results in EGR contentvariations as a function of crankshaft angle, causing the six differentcylinders to be filled with gas from the inlet box with an EGR contentwhich varies depending on whether the EGR flow is added to the fresh airby three or six pulses during a complete working cycle (two enginerevolutions). In other words, it is advantageous for both the turbineand the EGR function when there are six equal pulses rather than limpingpulsation. These six equal pulses are preferably provided by the exhaustflow from the six different cylinders being led together without anylong pipes and without dividing walls separating the exhaust flows fromthe various cylinders.

An example is described below of how an exhaust manifold meets all theabove requirements so as to avoid adding unburnt fuel from latepost-injections and/or deliberately heated exhaust gases to the EGRcircuit, and at the same time so as to provide six approximately equalpressure pulses for the turbine and EGR outlets on the exhaust manifold.

FIGS. 2 and 3 schematically depict two different examples of embodimentsof an exhaust manifold 14 with definitions of the partial volumes usedin the equations set out below. Thus V_(slag) denotes the swept volumeof each of the cylinders 12. V_(port) denotes the volume of therespective exhaust port in the cylinder head 29 up to the individualbranch 30 of the exhaust manifold. V_(g) denotes the volume of theindividual branches 30 up to the common portion of the manifold 14. Thiscommon portion of the manifold 14 is itself divided into a firstcollective subvolume V_(s1) which connects both to three of said branchvolumes V_(g) situated on one side of the exhaust manifold and to theoutlet 31 to the exhaust system, a second collective subvolume V_(s2)which connects both to at least one of the other branch volumes V_(g)and to the outlet 32 to the EGR circuit, and a third collectivesubvolume V_(sam1) which both connects to the remaining branch volumesand links the first and second collective subvolumes V_(s1), V_(s2). Themass flow through the outlet 31 to the exhaust system is denoted asm_(avg), and the mass flow through the outlet 32 as m_(egr).

The arrangement according to the invention is based on calculations ofcylinder production of exhaust flow at the six different inlets (fromthe respective cylinders) and the two different outlets on the exhaustmanifold, calculations which entail advanced CFD (computational fluiddynamics) simulations involving the geometry of the exhaust manifold andthe flow and pressure pattern during a whole working cycle of theengine, i.e. two revolutions. These advanced simulations cannot bereproduced in detail for every conceivable possible exhaust manifoldconfiguration, so what is disclosed below is merely a greatly simplifiedapproximate representation, in the form of two equations (1) and (2) forhow various exhaust manifold subvolumes, EGR contents and pressurepulses relate to one another with a view to fulfilling the condition ofpreventing the addition of unburnt fuel (late post-injection intendedfor post-treatment systems) from one or more of the three cylinders 1 to3 in one half of a six-cylinder engine, or with a view to deliberatelyensure that increased exhaust temperature from one or more of thecylinders in that group reaches only the turbine (the post-treatmentsystem) and not the EGR cooler. The EGR circuit is connected to themanifold in the vicinity of cylinder 5 or 6 at the other end of theengine.

The presuppositions are that m_(egr)/(m_(egr)+m_(avg))

When any of cylinders 4, 5 or 6 produces an exhaust pulse, the EGRcircuit is regarded as being supplied with gas from any of these threecylinders. For the EGR circuit not to be supplied with gas fromcylinders 1, 2 or 3 when they produce their exhaust pulse, equations (1)and (2) need to be fulfilled

$\begin{matrix}{{\frac{\left( {V_{{sam}\; 1} + {V_{5}/2} + V_{6}} \right)}{V_{slag}} - \frac{{megr} \cdot \left( {1 + {p_{\max}/p_{\min}}} \right)}{\left( {m_{egr} + m_{avg}} \right) \cdot 4}} > {\frac{p_{\max}}{p_{\min}}\frac{\left( {{V_{5}/2} + V_{6}} \right)}{V_{slag}}}} & (1) \\{\frac{V_{{sam}\; 1}}{V_{slag}} > \frac{m_{egr}}{m_{egr} + m_{avg}}} & (2)\end{matrix}$

Equation (1) refers to the situation at a pressure dip immediately aftera pressure peak (see diagram in FIG. 5) caused by an exhaust pulse fromcylinder 4, 5 or 6. Moreover, the amount of gas in subvolumes V_(sam1),V₆ and V₅/2 during a pressure dip

P_(min)·(V_(sam1)+V₅/2+V₆)   (1a)

is assumed to be sufficient to provide the EGR circuit with EGR gas

$\begin{matrix}\frac{{megr} \cdot {Vslag} \cdot \left( {P_{\min} + P_{\max}} \right)}{\left( {m_{egr} + m_{avg}} \right) \cdot 2 \cdot 2} & \left( {1b} \right)\end{matrix}$

from the pressure dip to the next pressure peak. At this pressure peakcaused by an exhaust pulse from cylinder 1, 2 or 3, the amount of gas

P_(max)(V₅/2+V₆)   (1c)

has to be a partial amount of the gas in (Ia).

In equations (1a) and (1c), volume V₄ is omitted, since it may beregarded as partly filled with gas from cylinders 1 to 3 during futurepressure rises due to exhaust pulses from cylinders 1 to 3, while volumeV₅ is divided by two since only to a small extent will it be filled withgas from cylinders 1 to 3 during future pressure rises due to exhaustpulses from cylinders 1 to 3.

From the bottom of the pressure graph to the top of the pressure graph,the EGR circuit is regarded as consuming EGR according to (Ib):

$\frac{m_{egr} \cdot V_{slag} \cdot \left( {P_{\min} + P_{\max}} \right)}{\left( {m_{egr} + m_{avg}} \right) \cdot 2 \cdot 2}$

where the first division by two is due to half a pressure pulse(one-twelfth of a whole working cycle) and the second division by two isdue to averaging of the two pressure levels p_(min) and p_(max).

After shortening by p_(min) and V_(slag), the result is (1a)−(1b)>(1c),from which the dimensionless equation (1) is derived.

Equation 2 is intended to compare the situation at a pressure low afteran exhaust pulse from cylinder 4, 5 or 6 with the situation at apressure low after an exhaust pulse from cylinder 1, 2 or 3.

Before an exhaust pulse from any of cylinders 1-3, the amount of gas in(2a):

P_(min)·V_(sam1)

has to be greater than the amount tapped by the EGR circuit during onepulse (one-sixth of a working cycle) according to (2b)

$V_{slag} \cdot p_{\min} \cdot \frac{m_{egr}}{m_{egr} + m_{avg}}$

After shorting by V_(slag) and p_(min), the result is therefore(2a)>(2b), from which the 15 dimensionless equation (2) is derived.

The cylinder numbering used above of 1 to 6 from front to rear may ofcourse be reversed, or the exhaust manifold 14 may be reversed so thatthe outlet for EGR is situated at the front end of the engine and theturbine outlet at the rear end of the engine (mirror reversal).Cylinders 1-3 may be the cylinders on one side (bank) of a V6 engine andcylinders 4-6 may be those on the other side (bank) of a V6 engine.

If the EGR outlet is relocated to a position between the fourth andfifth exhaust branches according to FIG. 3, equation (1) is modified toequation (3).

In the case of an exhaust manifold configured according to FIG. 3, theequation (3) set out below, in combination with equation (2), governshow the various exhaust 25 manifold subvolumes, EGR contents andpressure pulses relate to one another with a View to fulfilling thecondition of preventing the addition of unburnt fuel (latepost-injection intended for an aftertreatment system) from one or moreof the three cylinders 1 to 3 in one half of a six-cylinder engine, orwith a view to ensuring that the intended increased exhaust gastemperature from one or more of cylinders one to three reaches only theturbine (the aftertreatment system) and not the EGR cooler.

$\begin{matrix}{{\frac{\left( {V_{{sam}\; 1} + {V_{4}/2} + V_{6}} \right)}{V_{slag}} - \frac{m_{egr} \cdot \left( {1 + {p_{\max}/p_{\min}}} \right)}{\left( {m_{egr} + m_{avg}} \right) \cdot 4}} > {\frac{p_{\max}}{p_{\min}} \cdot \frac{\left( {{V_{4}/2} + V_{6}} \right)}{V_{slag}}}} & (3)\end{matrix}$

CFD calculations with an engine arrangement according to the inventioncorresponding to FIG. 2 result in the following proportionalrelationships between the exhaust gases at the turbine outlet 31 and theEGR outlet 32 respectively:

from cylinder 6: 19.2% turbine outlet, 80.8% EGR outlet

from cylinder 5: 34.8% turbine outlet, 65.2% EGR outlet

from cylinder 4: 45.8% turbine outlet, 54.2% EGR outlet

from cylinder 3: 99.7% turbine outlet, 0.3% EGR outlet

from cylinder 2: 99.6% turbine outlet, 0.4% EGR outlet

from cylinder 1: 99.0% turbine outlet, 1.0% EGR outlet

Consequently the EGR outlet 32 does not receive more than, at most,about 1% of the exhaust gases from cylinders 1 to 3, which can thereforebe used in the regeneration of, for example, a particle filter 23, forsupply of unburnt fuel by post-injection via existing injectors.

The invention is not to be regarded as limited to the embodimentexamples described above, instead a number of further variants andmodifications are conceivable within the scopes of the claims set outbelow.

1. An arrangement for controlling exhaust pressure pulsation of acombustion engine comprising an engine comprising six cylinders, aninlet for supply of air to the cylinders, and an exhaust manifold fordelivery of exhaust gases from the cylinders, the arrangement comprisinga first outlet to an exhaust system and a second outlet to a conduit forfeeding exhaust gases back, via an EGR circuit, from at least one of thecylinders of the engine to the inlet, at least one of the cylindersbeing arranged for addition of means for regeneration of an exhaust gasaftertreatment unit situated in the exhaust system, the exhaust manifoldcomprising branch volumes each appurtenant to a respective cylinderport, a first collective subvolume connecting both to three of thebranch volumes situated on one side of the exhaust manifold and to thefirst outlet to the exhaust system, a second collective subvolumeconnecting both to at least one of the other branch volumes and to thesecond outlet to the EGR circuit, and a third collective subvolume whichboth connects to remaining branch volumes and links the first and secondcollective subvolumes, wherein a mutual relationship between a sweptvolume of the cylinders, the exhaust manifold volume and the thirdcollective subvolume, the ratio of mass flow of EGR to the mass flow ofexhaust gases and the ratio between pressure maximum and pressureminimum in a pulsating exhaust flow, are such that EGR gas is separatedfrom regeneration gas in the exhaust manifold.
 2. An arrangementaccording to claim 1, wherein means for regeneration of an exhaustaftertreatment unit situated in the exhaust system comprises unburnthydrocarbon added in the cylinder after normal combustion.
 3. Anarrangement according to claim 1, wherein means for regeneration of anexhaust post-treatment unit comprises one of deferred combustion and,after normal combustion, addition of burnt hydrocarbon to increaseexhaust temperature.
 4. An arrangement according to any claim 1, whereinthe exhaust aftertreatment unit comprises a particle trap.
 5. Anarrangement according to claim 3, wherein the exhaust post-treatmentunit comprises a NOx trap/NOx catalyst.
 6. An arrangement according toclaim 1, wherein the exhaust system comprises at least one turbo unitcomprising a turbine for absorbing energy from the exhaust gases and acompressor for compression of air supplied to the inlet of the engine.7. An arrangement according to claim 1, wherein the engine is a straightsix-cylinder engine.
 8. An arrangement according to claim 1, wherein theengine is a V6 engine.
 9. An arrangement according to claim 1, whereinthe second collective subvolume connects to one of the other branchvolumes.
 10. An arrangement according to claim 1, wherein the secondcollective subvolume connects to two of the other branch volumes.